Sclerotherapy for Temple Veins?

Question about Sclerotherapy for Temple Veins: If the veins on the temple drain from the scalp? This means they are outside the scull? Then if back flow did happen how could the blood get inside the scull to cause problems? Also is it true that veins are not connect to arteries? If so how could the Sclerotherapy get into an artery to cause problems? Also if the veins drain from the scalp and do not feed the eyes then how could the Sclerotherapy cause vision problems?